CI should fail if warnings are generated during the build

[kim-em]

See https://github.com/leanprover/doc-gen4/actions/runs/12607913996/job/35139815612#step:3:79, which resulted in us merging #255 (and tagging v4.16.0-rc1) incorrectly.

[hargoniX]

As pointed out here: https://leanprover.zulipchat.com/#narrow/channel/113488-general/topic/Lean.20action.20not.20failing.20on.20.60lake.20build.20--wfail.60.20on.20docgen/near/491977688 #257 cannot fully address this issue as --wfail does not fail on configuration warnings.